In an inkjet printing system a carriage is moved in a scanning movement over a recording medium. The carriage comprises at least one inkjet print head. Typically the velocity of a moving carriage is determined by measuring the movement of the carriage with respect to a reference pattern, for example a linear code strip. The code strip is fixed to the carriage support frame and extends in the scanning direction of the carriage. Such a code strip is known for example from EP 1674278.
The reference pattern comprises a plurality of markers, the markers usually being mutually equidistant. The resolution of the reference pattern is selected in accordance with the criteria of the printing application and in particular in accordance with the printing resolution of the image. Typically for a print resolution in the scanning direction of, for example, 600 dots-per-inch (dpi) a reference pattern is used, which reference pattern has a marker resolution of a division sum of the print resolution divided by an integer number.
The markers of the reference pattern are detectable by a sensor. Said sensor is fixed to the carriage and is arranged such that it is able to sense the markers of the reference pattern. The sensor provides a sensor signal based upon the detection of the markers in the reference pattern.
The frequency that the sensor senses a marker depends on the movement of the sensor relative to the markers in the reference pattern. The marker frequency is reduced in case the movement of the object is decelerated. As a result the information frequency in the sensor signal is reduced. Consequently the frequency of determining the velocity decreases when the velocity of the carriage decreases.
In any position of the object, where the sensor does not sense a marker of the pattern, the actual velocity and position of the object is unknown. In known printing systems the position and velocity of a carriage when moving between subsequent markers is estimated based upon a previous determination of the velocity of the object.
Thus the frequency of determining the velocity effects the accuracy of the determination of the velocity. And a lower frequency reduces the accuracy of the determination.
The disadvantage of methods for determining a velocity of an object in a printing system is the inaccuracy in determining the velocity of the object, in particular when the object is moving slow relatively to the reference pattern.